On a road, once you go past this sign, can you drive at a speed of 20 km/h?

In the video you turn into a small estate street, at the entrance to which there is sign D-40 "residential area". On the road beyond this sign you can drive at 20 km/h. The answer is: yes.

The Road Traffic Act states that the speed limit in a residential area is 20 km/h, and the Regulation on road signs and signals specifies that sign D-40 marks the entrance to a residential area in which devices that force slow driving, for example speed bumps, might not be marked with warning signs. 20 km/h is also the maximum - driving at 30 km/h would already be a petty offence. In this area a pedestrian may walk in the middle of the roadway and has right of way over vehicles, and parking is allowed only in marked places, so you drive slowly, in a low gear, ready to stop at any moment.

Legal basis: Art. 20 para. 2, Art. 11 para. 5 and Art. 49 para. 2 point 4 of the Road Traffic Act (Prawo o ruchu drogowym); § 58 para. 1 and 2 of the Regulation on road signs and signals